Typical underground electrical cables include a number of copper or aluminum strands surrounded by a semiconducting or insulating strand shield, a layer of insulation, and an insulation shield. This design of underground cables is known for having a useful life of 25-40 years. In some instances, the life span of an underground cable is shortened when water enters the cable and forms micro-voids in the insulation layer. These micro-voids spread throughout the insulation layer in a tree like shape, collections of which are sometimes referred to as water trees.
Water trees are known to form in the insulation layer of electrical cables when medium to high voltage alternating current is applied to a polymeric dielectric (insulator) in the presence of water and ions. As water trees grow, they compromise the dielectric properties of the polymer until the insulation layer fails. Many large water trees initiate at the site of an imperfection or a contaminant, but contamination is not a necessary condition for water trees to propagate.
Water tree growth can be eliminated or retarded by removing or minimizing the water or ions, or by reducing the voltage stress. Another approach requires the injection of dielectric enhancement fluid into interstices located between the strands of the cables. Please see U.S. Pat. No. 5,907,128 for a more detailed description of such an approach. The fluid reacts with water inside the cable and oligomerizes to slow diffusion and provide long lasting dielectric enhancing fluid. The oligomerized fluid functions as a water tree retardant and provides other beneficial properties.